Increased acquisition in the Step/Phambili trials, coupled with the lack of efficacy in HVTN-505 indicate that vaccine-elicited CD4+ T cell responses can mitigate protection and, in some cases, increase acquisition. This frames a major issue in HIV vaccine development; how to elicit the CD4+ T cell responses necessary for B cell help without increasing the numbers or phenotypes of target cells for HIV infection that attenuate protection? Early studies in non-human primates (NHPs) showed that immunization with a varicella zoster gp120-SIV (VZV-gp120) dramatically increased SIV replication, prefiguring the Step/Phambili trials. NHP studies following the AdHu5 efficacy trials confirmed that AdHu5 immunization increases transmission of SIV, probably due to the increase of mucosal CD4+ CCR5+ T cells that appears to be a general property of this vector system. In this Program, we show that the ability of vaccine-elicited CD4+ T cells to attenuate protection extends to other vector systems and to subunit protein-adjuvant formulations in NHPs. Currently, there is no direct test of the hypothesis that vaccine elicited CD4+ T cells can attenuate antibody-mediated protection. This gap in knowledge is a key problem in HIV vaccine development. Our long-term goal is to develop an effective HIV vaccine, which is supported by our overall objective in this project to determine whether vaccine-elicited CD4+ T cells play a causal role in attenuating antibody-mediated protection. The central hypothesis of this project is that vaccine-elicited CD4+CCR5+ T cells can attenuate passive protection by a broadly neutralizing monoclonal antibody. This hypothesis was developed pursuant to NHP protection studies where inverse correlations between protection in repeat low-dose challenge studies and levels of vaccine-elicited CD4+ T cells are observed, even at putative protective antibody titers. Our approach will employ a new glycan-shield bnmAb, N60-B1.1, which passively protects NHPs against a rectal high-dose challenge with SHIV162P3. N60- B1.1 will be used to test whether vaccine-elicited CD4+ CCR5+ T cells attenuate its protective efficacy. The rationale for the proposed research is that if vaccine-elicited CD4+ T cell responses attenuate N60-B1.1 protection, causality will be demonstrated between such responses and attenuated antibody-mediated protection. Testing our central hypothesis and thereby accomplishing our overall objective will be pursued via three specific Aims. Aim 1 will titrate N60-B1.1 to define a dose that affords optimal sensitivity for attenuation by vaccine-elicited CD4+ T cell responses. Aim 3 will directly test the hypothesis that AdHu5 vector-elicited CD4+ T cells attenuate the efficacy of passively administered N60-B1.1 against a high-dose rectal challenge with SHIV162P3. This will be accomplished using an AdHu5-SIV-Gag/Tat immunogen that is known to elicit high levels of rectal CD4+ CCR5+ T cells. Aim 3 will test the hypothesis that subunit/adjuvant-elicited CD4+ T cells attenuate the efficacy of passively administered N60-B1.1. This will employ a Gag/Tat subunit protein formulated in Iscomatrix, an adjuvant that our preliminary data shows favors acquisition over protection.